1. Field of the Invention
The invention relates to a sound-producing device including an acoustical waveguide and an acoustical generator coupled to said waveguide.
It applies to all electroacoustical fields, including high fidelity.
The invention is more particularly concerned with the shape of the waveguide forming the acoustical horn with the aim of obtaining good control of the dispersion of the sound by means of a relatively compact and in particular relatively shallow system.
2. Description of the Prior Art
In producing sound, good control of the dispersion of the sound by an acoustical generator conventionally imposes the use of a horn forming a large acoustical waveguide. Consequently, a box forming an acoustical enclosure and enclosing at least an acoustical generator and its waveguide is generally bulky, and in particular relatively deep, since the depth of said acoustical enclosure depends essentially on the length of the horn.
French patent No. 88-02481 defines an acoustical generator associated with an acoustical waveguide. Obstacles between the entry and the exit of the waveguide are shaped to homogenize the acoustical paths between the entry and the exit of the waveguide. The wavefront obtained is rectangular and has a straight profile.
U.S. Pat. No. 5,900,593 uses similar principles but additionally a mirror in the form of a curved dihedron of circular arc shape adapted to modify the sound propagation direction. The wavefront obtained is rectangular and has a convex profile.
A first object of the invention is to form an acoustical wavefront of chosen shape and having a convex, concave or plane profile by means of a small waveguide.
Coupling a plurality of conventional sound-producing devices leads to irregularities in the dispersion of the sound due to the occurrence of acoustical interference between the sound waves issuing from the various waveguides.
A second object of the invention is to propose an arrangement of sound-producing devices enabling several devices to be coupled together in such a manner as to allow good control of the shape of the acoustical wavefront emitted by the set of acoustical generators without creating troublesome interference.
The invention is based on the principles of geometrical acoustics, i.e. the field of acoustics based on ray theory. It therefore applies laws known from optics to the propagation of sound, in particular the laws of reflection of rays from conic section surfaces. By xe2x80x9cconic section surfacexe2x80x9d is meant a surface generated by rotating a curve from the conic family. More particularly, in the context of the invention, advantageous acoustical properties have been discovered and put to use that are associated with acoustical reflections from surfaces such as hyperboloids, paraboloids or ellipsoids.
The basic principle of the invention resides in the fact that using a reflection surface of the above kind as an acoustical mirror makes it possible to displace the apparent point of emission of a sound source.
The invention relates more particularly to a sound-producing device including at least one acoustical generator and an acoustical waveguide provided with an entry to which the acoustical generator is connected and an exit of chosen shape from which an acoustical wave propagates to the outside, wherein the waveguide includes two duct sections, namely a first section aligned with the exit and a second section aligned with the inlet, the two sections are connected partly by a curved reflecting surface having substantially the shape of part of a conic section surface, and the entry is defined in the vicinity of a focus of said conic section surface.
Note that the waveguide as described can be perfectly extended by another horn.
The interior volume of the first section is preferably substantially delimited by the intersections of:
the surface of the exit,
a first lateral surface generated by a generatrix passing through a first focus of the conic section surface and resting on the contour of the exit, and
the curved reflecting surface delimited inside a contour defined by the intersection of the conic section surface and the first lateral surface.
Similarly the internal volume of the second section is substantially delimited by the intersection of a second lateral surface generated by a generatrix passing through a second focus of the conic section surface and resting on the contour of the reflecting surface and the reflecting surface itself, excluding the volume portion shared with the first section.
In defining said first and second sections, it amounts of course to totally the same thing to consider that the internal volume of the second section includes the common portion and that the latter is subtracted from the internal volume of the first section. The geometrical definition of the first and second sections of the acoustical waveguide is no more than a convenient means of describing the overall shape of the internal volume of the waveguide.
The inlet is defined in the vicinity of the second focus. Because the sound source cannot be a point source, the second section includes, in the vicinity of the second focus, a widened mouth connected to the second lateral surface. The mouth has a shape and dimensions suited to the attached acoustical generator.
If the conic section surface is a hyperboloid, it is just as if the sound were emitted from the first focus, which is to the rear of and at a distance from the component parts of the device. Accordingly, in this case, the depth of a sound-producing device can be significantly reduced compared to what it would be if an acoustical horn were entirely formed between the first focus and the aforementioned exit. What is more, this configuration facilitates coupling a plurality of similar devices in order to emit a convex wavefront without creating interference between the sources.
If a paraboloid is used, the first focus is projected to infinity behind the mirror and the acoustical wavefront is plane. This type of emission is equally beneficial for homogenizing sound propagation in a room and for achieving good coupling between a plurality of sources without interference.
Finally, if the mirror is a portion of an ellipsoid, the first focus is shifted to the front of the opening so that the sound appears to be created at a given point in the listening room. The wavefront is concave. A plurality of similar devices can likewise be coupled without interference, producing the effect of a virtual sound source in the listening room.
The invention will be better understood and other advantages of the invention will become more clearly apparent in the light of the following description of various embodiments of a sound-producing device according to the invention, which description is given by way of example only and with reference to the accompanying drawings.